Pentanoic acids, also called valeric acids, have gained economic significance as intermediates in industrial organic chemistry. They occur in four different structural isomers as linear n-pentanoic acids, branched 2-methylbutyric acid, branched 3-methylbutyric acid and highly branched pivalic acid. Pentanoic acids can be used as such, for example for the production of fragrances. In addition, esters of pentanoic acids with polyols are finding increasing significance as lubricants. The pentanoic acids can be used in pure isomeric form or in the form of an isomer mixture (Weissermel, Arpe, Industrielle Organische Chemie [Industrial Organic Chemistry], 3rd edition, VCH Verlagsgesellschaft mbH, Weinheim, 1988, page 218; Schneidmeir, Chemiker-Zeitung, volume 96 (1972), no. 7, pages 383-387). The isomeric pentanoic acids are produced industrially by oxidation of the corresponding pentanals, or in the case of pivalic acid by hydrocarboxylation (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, vol. 6, pages 495-503).
The pentanal starting compounds are prepared industrially by reaction of butenes with synthesis gas, a mixture of carbon monoxide and hydrogen, in the presence of transition metal compounds. The reaction of olefins with synthesis gas is also referred to as the hydroformylation reaction or oxo reaction, and the hydroformylation of but-1-ene affords, as well as the straight-chain n-aldehyde n-pentanal, also certain proportions of the isoaldehyde 2-methylbutanal (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, vol. 2, pages 73-74; vol. 25, pages 286-289).
Butenes are obtained industrially by the steamcracking of naphtha. Typically, separation of 1,3-butadiene from the butene cut from the naphtha cracking to form raffinate I is followed by separation of isobutene to form raffinate II (Weissermel, Arpe, Industrielle Organische Chemie, 3rd edition, VCH Verlagsgesellschaft mbH, Weinheim, 1988, pages 71-79). For the subsequent hydroformylation reaction, predominantly raffinate II is used, in which a small residual isobutene content can be permitted. In special cases, it is also possible to process raffinate I having a high isobutene content, and occasionally also a but-1-ene-depleted raffinate II, which can also be referred to as raffinate III.
The hydroformylation reaction can be conducted either in the presence or in the absence of complex-forming compounds, for example in the presence of organophosphorus compounds. According to EP 0 366 089 A2, a homogeneous organic solution is employed with rhodium triphenylphosphine catalysis. Since a maximum proportion of n-pentanal compared to 2-methylbutanal in the pentanal mixture formed is generally the aim, the hydroformylation reaction is frequently conducted in the presence of homogeneously dissolved transition metal complexes, which first enable isomerization of the but-2-ene to but-1-ene, which is then hydroformylated predominantly to n-pentanal. Rhodium complex catalysts suitable for the isomerizing hydroformylation of a mixture of linear butenes are described, for example, in DE 102 25 282 A1, in which the complex ligands have a xanthene skeleton.
Rhodium complex catalysts based on bisphosphite ligands together with sterically hindered secondary amines, which are likewise suitable for the isomerizing hydroformylation of a mixture of linear butenes, are discussed in DE 10 2008 002 187 A1. Two-stage process variants are also known, for example according to DE 43 33 324 A1, DE 42 10 026 A1, DE 101 08 474 A1 and DE 101 08 475. In the first stage, preferably but-1-ene is converted, while, in the second stage, the but-2-ene-containing offgas from the first stage is hydroformylated to give a mixture of n-pentanal and 2-methylbutanal. According to DE 43 33 324 A1 and DE 101 08 474 A1, the first hydroformylation stage can also be conducted in the presence of water-soluble rhodium complex catalysts. In this type of reaction regime, a liquid aqueous catalyst solution is present alongside the liquid organic reaction solution, which, after leaving the hydroformylation zone, can be separated from one another in a simple manner by phase separation. Because of the presence of an aqueous phase and an organic liquid phase, this type of reaction regime is also referred to as a heterogeneous process or biphasic process.
According to the composition of the butene feed mixture and the reaction conditions in the hydroformylation stage, a pentanal mixture is obtained with varying proportions of n-pentanal, 2-methylbutanal, 3-methylbutanal and a small amount of pivalaldehyde, which is typically distilled. Because of the small difference in boiling point between 2-methylbutanal (92° C. at standard pressure) and 3-methylbutanal (92.5° C. at standard pressure), 3-methylbutanal cannot be separated completely from the 2-methylbutanal with acceptable distillation complexity and the 2-methylbutanal obtained is used for the preparation of 2-methylbutyric acid with a residual 3-methylbutanal content of typically greater than 0.2% by weight, based on the organic component. For particular applications, for example for fragrance production or for specific lubricants, however, there is a demand for a 2-methylbutyric acid quality where the residual 3-methylbutyric acid content has to be below 0.2% by weight, based on the organic component. A very low residual content of 3-methylbutyric acid below 0.2% by weight may also be important for n-pentanoic acid/2-methylbutyric acid mixtures. There is therefore a need for a process for preparing 2-methylbutyric acid having a reduced residual content of 3-methylbutyric acid from the secondary streams obtained in the preparation of pentanoic acids.